The DNA damage response and DNA repair defects found in cells with a deficiency of the BRCA1 or BRCA2 protein have been extensively characterized in the last decade. However, the connections between the two proteins remain relatively poorly understood, as they function at different steps in the pathway of homologous recombination. The major role of the BRCA1-BRCA2 pathway of homologous recombination (HR) is to protect the genome from errors arising out of endogenously created double-strand breaks, which are created during DNA replication. BRCA1 is activated to promote homologous recombination by a critical post-translational modification by Chk2 on serine 988, which then results in controlling the extent of 5'-end resection at double- strand breaks. The first aim will focus on how BRCA1 controls both end resection and the subsequent recruitment of downstream factors in homologous recombination including BRCA2. The second aim is specifically focused on what happens to double-strand breaks, created during replication or exogenously, in the absence of BRCA1 or BRCA2. The Rad52 protein is redundant for homologous recombination in mammalian cells, but in the absence of a functioning BRCA1-BRCA2 pathway, it becomes critical for cell viability. The goals of this second aim are to understand the Rad52-Rad51 pathway in mammalian cells as it may give insight on how tumor cells defective in BRCA1-BRCA2 pathway survive normal DNA replication. Rad52 is therefore a key tumor-specific target for therapy in BRCA-deficient tumors. The third aim plans to look at new connections in the BRCA1-BRCA2 pathway that are defective in sporadic breast cancers, where there is no genetic defect in BRCA1 or BRCA2, but the pathway is not connected. A failure to recruit BRCA1 to sites of DNA damage is observed and the road-block appears to occur in the upstream DNA damage pathway to BRCA1 between RAP80 and Abraxas. All three aims are focused on the connections in the BRCA1-BRCA2 pathway of homologous recombination, and the knowledge from these proposed experiments should allow new insight for treating human cancers with defects in this pathway.